Neutrinos are everywhere. They permeate the very space all around us. They can be found
throughout our galaxy, in our sun and every second tens of thousands of neutrinos are passing
through your body. But there is no need to become alarmed for these tiny particles barely
interact with anything. In fact, they can even pass through the entire Earth without being affected.

Neutrinos are fundamental particles that were first formed in the first second of the
early universe, before even atoms could form. They are also continually being produced in the
nuclear reactions of stars, like our sun, and nuclear reactions here on earth. Much is still
unknown about these particle, they have an undetermined mass and travel at near the speed of
light.

There are three types of neutrinos: electron neutrino, muon neutrino, and tau neutrino.
According to the standard model ( see figure 1) there exist 12 fundamental particles. Each
"flavor" of neutrino has a corresponding charged particle from which it gets its name. The
Standard Model consists of three generations and each generation has two quarks a neutrino and a
charged particle.The particles in the standard model are separated into two types: quarks and
leptons. The quarks interact via the strong nuclear force while the leptons interact via the
electromagnetic or the weak nuclear force. Neutrinos are nearly massless and have no electric
charge. Therefore, unlike the other particles, they only interact via the weak nuclear force.
Neutrino actually means "little neutral one." Since the weak nuclear force only acts at shot
ranges, neutrinos can pass through massive objects without interacting with them.

History

Because of the neutrinos' elusive behavior, their existence was not even known until
1959 even though they had been predicted back in 1931. Wofgang Pauli first predicted the neutrino
in order to account for the apparent loss of energy and momentum that he observed when studying
radioactive beta decays(see Figure 2).

Fig. 2 The neutron Decay.(Neutrinos)

He predicted that the energy was being carried off by some unknown particle. Then
in 1959, Clyde Cowan and Fred Reines finally found a particle that fit the description of the
proposed neutrino by studying the particles created by a nuclear power plant. By doing this they
actually discovered the electron neutrino. The next big discovery was that of the muon neutron
found by Leon Lederman, Mel Schwartz, and Jack Steinberger, scientists at CERN. They did this by
firing a GeV proton beam through a target thus producing pions, muons, and muon neutrinos.

The first experiment to attempt to detect electron neutrinos from the sun was conducted
by a detector in the bottom of the Homestake mine in South Dakoda in 1968. However they detected
only neutrinos about twice a week. It was predicted however that the detector should find about
one of the 1016 solar neutrinos a day. This unexplainable lack of solar neutrinos detected
became known as the Solar Neutrino Problem. It is thought that the neutrinos actually oscillate
between the different "flavors" after being emitted from the sun as electron-neutrinos. Therefore
they were not detecting all of the neutrinos because some had changed into muon and tau neutrinos.

The existence of the third flavor of neutrino, the tau neutrino, was first inferred in
1978 with the discovery of the Tau particle at SLAC, the Stanford Linear Accelerator Center. They
realized that the Tau particle was just a heavier version of the electron and muon and therefore
should have a corresponding neutrino as well. The tau neutrino evaded detection though for many
years. Firstly the Tau particle only lasts for about 300 fs, making them difficult to track and
therefore making it difficult to track their corresponding neutrino. Secondly, tau neutrinos are
incredibly rare. However, in 2000 the scientists at CERN on the DONUT detector were finally able
observe a tau neutrino.

Where Neutrinos Come From and Why They Are So Cool

A star implodes in a cataclysmic bust of energy, a scientist measures the small energy
discrepancies of radioactive decay, the sun constantly baths us with warmth and showers us with
particles, all around us the universe is saturated with the remnants of the Big Bang, and in all
of these things there is one thing in common: neutrinos. Although they are hard to detect, these
little particles can tell us about everything from the birth of the universe to the nuclear
reactions that power our cities.

Supernova

On February 23 1987 detectors deep underground that where designed to detect proton
decay suddenly detected a huge number of neutrinos (8 in 5 seconds). Scientist where perplexed
by this influx in neutrinos at first until on February 24 a grad student named Ian Shelton
announced his observation of a supernova in the Large Magellanic Cloud. This was a core collapse
supernova. When the core of a massive star collapses, it crushes the protons and electrons
together and neutrinos form.

p + e → n + ν.

The neutrinos pass straight through the collapsing star before the explosion takes
place. This is why the neutrinos where detected before the supernova was visibly observed.
When the neutrinos leave, they also take energy away from the star and the star continues to
collapse and rebounds out in an explosion that can outshine the brightness of the entire galaxy.
Neutrinos are very important to the study of supernovas because they provide an early warning
signal and allow scientists to be looking in the right direction before the supernova even takes
place.

The Sun

Neutrinos are also created in the nuclear reactions that power the core of stars like
our sun. Neutrinos are formed in the proton- proton chain.

p + p → deuteron + positron + neutrino,

where the deuteron is the nucleus of deuterium. In the sun, 4 hydrogens are being fused
into Helium by means of the proton-proton chain. Neutrinos are important because they allow
scientists to peek into the interior of the sun and learn about the processes there. All other
information about the sun is from electromagnetic radiation that has to pass through the many
layers of the sun interacting and changing along the way before traveling through space to us.
This whole process can take up 105 to 106 years. However, the neutrinos pass cleanly
through the sun in a few seconds without interacting and take a mere 8 minutes to travel from the
core where they are created to us.

The Big Bang

The greatest source of neutrinos happened some 15 billion years ago. The neutrino was
first created 10-4 seconds after the big bang. Then at only 1 second after the big bang the
universe became transparent to the neutrino allowing them to travel freely through space. At this
time the universe had a temperature of about 3*1010. Since the time of the big bang the
universe expanded and cooled and continues to expand to this day. There are about 330 million of
these neutrinos per m3; however, these neutrinos have very low energy. They form a cosmic
background radiation that is only 2.73 degrees Kelvin today. By studying these neutrinos
scientists are able to learn about the universe when it was forming.

The Future of Neutrino Research

Scientists are constantly coming up with new and ingenious ways to study neutrinos
from space. Neutrino telescopes like Super-Kamiokande in Japan use huge vats of water to detect
neutrinos (see Figure 3).

Fig. 3 The Super-Kamiokande neutrino telescope (Casper,Dave)

The inside of the tank is lined with 11,146 photo-multiplier tubes that detect
Cherenkov light. Cherenkov light is emitted by the neutrinos as they pass through the water at
near the speed of light.Therefore the detector detects the effect of the neutrinos
interacting with the water and not the neutrinos themselves. Telescopes like the Super-Kamiokande
are deep underground in order to avoid detecting other particles from cosmic rays. The Homestake
telescope uses Chlorine 37 coming from Argon and the GALLEX telescope uses Germanium 71 coming
from Gallium as the medium to detect neutrinos instead of water.

Currently, scientists are building a better neutrino telescope by using the
clear polar ice as a medium by which to detect the neutrinos. IceCube is a one-cubic-kilometer
new neutrino telescope being built in the South Pole (see Figure 4). It will be an array of 80
strings, each string having 60 optical moduals that are desigened to detect the cherenkov light
from emitted from muons, which are a byproduct of the neutrinos interacting with the ice.
Scientists are building this new telescope under the South Pole because it allows them to make it
incredibly large, to have a very stable place for the detectors, to keep a stable temperature, and
to built it deep enough to avoid interference from cosmic rays.

Fig. 4 The IceCube neutrino telescope (IceCube)

The arctic ice also makes a good medium because it is pure, transparent, and free from
radioactivity. Scientists hope that by building IceCube they will be able to learn more about far
away neutrino sources like gamma ray bursts, supernova, black holes and maybe even dark matter.